Gabriel Christian Natale-Hjorth, 19, and Finnegan Lee Elder, 20, have been imprisoned in the Italian capital since July 2019 on suspicion of aggravated murder and attempted extortion.
The pair is accused of killing Mario Cerciello Rega, a 35-year-old Italian police officer who was stabbed 11 times in the early hours of July 26 after a drug deal gone wrong.
The San Francisco natives, who had been on holiday in Rome and were teenagers at the time, face a life sentence if convicted.
The murder shook Italy and thousands attended the funeral of Cerciello Rega, who had just returned from his honeymoon when he was killed.
“His murder cannot and must not go unpunished,” the victim’s wife, Maria Rosaria Esilio, said in a statement on the eve of the trial.
“My family has ended with Mario’s death, because our unborn children and dreams were buried with him.”
Presiding over the trial will be two judges and a six-person jury, as well as two reserve jurors, from the general public, who will decide on the case, which is expected to take many months of deliberations.
Both young men were in court on Wednesday, sitting near their lawyers with a pair of penitentiary police officers standing behind them.
Elder’s parents sat in the back row; the slain officer’s widow sat in the second row.
“We are grateful that this process is finally moving forward and we hope that this trial can stay focused on the facts of what happened that night,” Elder’s family said in a statement.
“We look forward to the truth coming out, and to Finn coming home.”
Italian investigators said Elder has admitted to stabbing Cerciello Rega, claiming he believed the victim and his partner were dangerous drug dealers.
Investigators allege that the American teens were trying to buy drugs in Rome but were sold a fake substance.
Elder and Natale-Hjorth then allegedly robbed the man that had directed them to the drug dealer, who had contacted the Italian police.
Cerciello Rega, along with his partner, responded to the call and were confronted by the suspects, with a scuffle ensuing that saw Elder allegedly stabbing the officer with an 18-centimetre serrated-edge combat knife, while Natale-Hjorth reportedly attacked the officer’s partner.
The pair were later arrested in the hotel room near the killing where they were staying after allegedly being caught on CCTV fleeing the scene, police said.
Investigators also said they found the knife used in the killing in the ceiling of the hotel room.
After several hours of interrogation and “faced with overwhelming evidence, they confessed”, according to the Provincial Command of Rome.
Shortly after the interrogation, Italian newspapers published a leaked photo, which appeared to show one of the suspects blindfolded and handcuffed, which prompted speculation that the pair may have confessed under duress.
However, an Italian prosecutor said the interrogation was conducted lawfully, in the presence of defense lawyers and interpreters, two days after the photo was published in July.
